The sleep of a human being is significantly closely related to a brain wave. Brain waves are classified into alpha waves (7-14 Hz), beta waves (14-30 Hz), delta waves (0.5-4 Hz), and theta waves (4-8 Hz) depending on activity states of a brain. When alertness level is excessively high like an exercised state or a nervous state, the theta waves may mainly appear. In a sleepy state or a sleep state, the delta waves may mainly appear. In the state that the alertness level is excessively neither high nor low, the alpha waves or the theta waves may appear.
During sleeping, sleep types are mainly divided into Rapid Eye Movement (REM) sleep and non-REM sleep according to the types of brain waves. The non-REM sleep is generally sub-divided into four stages
When a person is in a wakes-up state, the beta waves are mainly observed. Thereafter, if the person closes eyes and feels comfortable, the alpha waves starts increasing and the person falls into non-REM sleep. As the stage of the non-REM sleep passes, if the person more deeply falls asleep, the brain becomes calm to emit the delta waves. In this case, among sleep patterns, if a specific stage excessively less appears or frequently appears, mental or physical problems may be caused.